A scissor gear assembly is well known in the industry and often used in combustion engines for driving cam shafts, balancer shafts, fuel injection pumps or air compressors to prevent rattling noise because of load reversals and/or crank torsion vibrations. The scissor gear comprises a main and an auxiliary gear rotatable on a mutual axis and in rotation direction connected by a resilient member. The resilient member can be a spring with an annular shape. Preferably the spring is selected in this way that the negative torque due to load reversals and crank vibrations are eliminated. The scissor gear assembly has contact with both the driving flank and coast flank of its mating gear and because of this rattling is prevented. Especially the rattling noise at low rpm for instance at idling is noticeable and not wanted. Because of inertia of the gears and of the rotating components the rattling at high rpm will disappear or will not be found disturbing. In some applications a higher spring torque at higher rpm is beneficial because of higher torsion vibrations at higher rpm.
A disadvantage of using a scissor gear is that the constant load of the spring, and the introduction of an additional gear mesh will increase the level of high frequent noise (other than rattling noise) or introduce a high frequent whine noise. A further disadvantage is that because of the friction between the first and the second gear the hysteresis of the resilient member and the extra roll friction of the complementary gear mesh will give also energy losses. The energy losses will depend from the amount of spring load of the resilient member. So the selection of the spring load should be balanced, on one hand the spring load should be high enough to eliminate or reduce rattling noise to an acceptable level and on the other hand the spring load should be not too high introducing high frequent noise and causing unnecessary energy lost. Ideally the spring load should be in function of the engine speed. For some applications the maximum amount of spring load is only necessary at low rpm's or at idle speed. So at high rotational speed the ideal spring load should be reduced or eliminated to have a more efficient transmission and to have less whine noise. In other applications, rattling is still an issue at higher rotational speed because of progressive torsion vibrations. For latter applications the ideal spring torque should be progressive with the rotational speed so the transmission is as efficient as possible and having acceptable noise performance.
For most applications the goal of the invention is to find a scissor gear wherein the spring delivers a negative torque to eliminate or reduce rattle noise and that is significantly degressive in function of the rotational speed. For some other applications the goal of the invention is to find a scissor gear wherein the spring delivers a negative torque to eliminate or reduce rattle noise and that is significantly progressive in function of the rotational speed.